The present invention relates to a continuous casting nozzle, in particular, a continuous casting nozzle which permits effective prevention of narrowing and clogging of the inner bore thereof through which molten steel passes in performing continuous casting of the molten steel containing aluminum such as aluminum-killed steel.
A continuous casting nozzle for casting molten steel is used for the following purposes.
A continuous casting nozzle has a function of pouring molten steel from a tundish to a mold. In continuously casting molten steel, a continuous casting nozzle is used for such purposes as preventing the molten steel from being oxidized by contacting with the open air, preventing the molten steel from splashing when the molten steel is poured from a tundish to a mold, and rectifying the flow of the poured molten steel so as not entrap non-metallic inclusion and slag present near or on the mold surface into the cast steel strand.
A refractory material of a conventional continuous casting nozzle of molten steel comprises graphite, alumina, silica, silicon carbide or the like, for example. However, there are following problems when aluminum-killed steel or the like is cast with the use of the conventional casting nozzle.
In casting the aluminum-killed steel or the like, aluminum which is added as a de-oxidizer, reacts with oxygen existing in the molten steel to produce non-metallic inclusion such as alpha (xcex1)-alumina or the like. In addition, when the molten steel flows through the nozzle, the aluminum in the molten steel reacts with oxygen in the open air to further produce alumina.
Therefore, in casting the aluminum-killed steel or the like, the non-metallic inclusion such as xcex1-alumina adheres and accumulates onto the surface of the inner bore of the continuous casting nozzle, so that the inner bore is narrowed or clogged up in the worst case so as to make the stable casting thereof difficult. Furthermore, the non-metallic inclusion such as xcex1-alumina adhered or accumulated onto the surface of the inner bore peels off or falls down, and the non-metallic inclusion thus peeled off or fell down is entrapped into the cast steel strand, thus degrading the quality of the cast steel strand.
In order to prevent the above-mentioned reduction or clogging of the inner bore of the nozzle caused by the non-metallic inclusion such as xcex1-alumina, there has widely been used the method in which inert gas is ejected from the inner surface of the nozzle bore toward the molten steel flowing through the inner bore so as to prevent the non-metallic inclusion such as xcex1-alumina existing in the molten steel from adhering or accumulating on the surface of the inner bore of the nozzle (for example, the method disclosed in Japanese Patent Publication No. Hei 6-59533/1994).
However, there are problems in the above-mentioned method in which the inert gas is ejected from the inner surface of the inner bore of the nozzle, as follows:
When a large amount of inert gas is ejected, bubbles produced by the inert gas is entrapped into the cast steel strand to cause pinholes in the cast steel strand, thus deteriorating the quality of the cast steel. On the other hand, when a small amount of inert gas is ejected, the sufficient effect of the inert gas is not obtained, and non-metallic inclusion such as the xcex1-alumina is adhered and accumulated onto the surface of the inner bore of the nozzle, thus causing narrowing or clogging, in the worst case, of the inner bore.
In addition, it is substantially difficult to manufacture the nozzle which enable to uniformly eject the inert gas from the surface of the inner bore of the nozzle toward the molten steel flowing through the inner bore. Furthermore, when the casting is performed for a long period of time, it becomes gradually difficult to stably control the amount of ejected inert gas, since the refractory material of the continuous casting nozzle degrades. As a result, the non-metallic inclusion such as xcex1-alumina adheres and accumulates onto the surface of the inner bore of the nozzle in such manner that the inner bore is narrowed or eventually clogged up.
The clogging of the nozzle by the non-metallic inclusion, particularly alumina (Al2O3) inclusion is deemed to be caused as follows:
(1) Aluminum in the molten steel is oxidized by the entrapped air which passes through a joint portion of the nozzle refractory and the refractory structure per se to produce alumina, or silica in the refractory including carbon is reduced to produce SiO which supples oxygen to produce alumina.
(2) Alumina inclusion is produced by diffusion and cohesion of the alumina produced in the above process.
(3) Graphite and carbon on the surface of the inner bore of the nozzle are taken away in such manner that the feature of the surface of the inner bore becomes rough, and thus the alumina inclusion is apt to accumulate on the rough surface of the inner bore.
There is proposed a nozzle as a remedy to solve the above problem, in which a non-oxide raw material (SiC, Si3N3, BN, ZrB2, Sialon, etc.) that has a low reactivity with aluminum oxide is added to alumina-graphite refractory, or a nozzle consisting of the above non-oxide material itself (for example, refer to Japanese Patent Publication No. Sho 61-38152/1986).
However, it is not practical to add the above non-oxide material to the widely used alumina-graphite refractory, because the effect of preventing adhesion is not recognized unless a large amount of the non-oxide material is added, and furthermore, the corrosion resistance thereof is deteriorated when a large amount of the non-oxide material is added thereto.
Also, the nozzle consisting essentially of the non-oxide material is not suitable for practical use, since the material cost and manufacturing cost are expensive, while the substantial effect of preventing adhesion may be expected.
There is further proposed a nozzle, the refractory thereof comprising graphite-oxide raw material containing CaO, in which an oxide raw material containing CaO (CaO.ZrO2, CaO.SiO2, 2CaO.SiO2, and the like) produces by a reaction of CaO with Al2O3 a low-melting-point material which is easily separated from the molten steel (for example, refer to Japanese Patent Publication No. Sho 62-56101/1987).
However, since the reactivity of CaO with Al2O3 is apt to be influenced by a temperature condition of the molten steel in casting, it is difficult to effectively produce the low-melting-point material. In addition, a large amount of CaO is required to supply when a large amount of Al2O3 inclusion is contained in the steel. However, it is difficult to contain sufficient amount of CaO in the refractory of the nozzle, since spalling resistance and corrosion resistance thereof are deteriorated. Furthermore, zirconia (ZrO2) is difficult to be separated from the molten steel, since zirconia in the aggregate flowing into the molten steel from the refractory has a high specific gravity so that zirconia stays in the molten steel.
The object of the present invention is to provide a continuous casting nozzle which may prevents alumina inclusion from adhering and accumulating on the inner surface of the nozzle, and prevents the inner bore of the nozzle from being narrowed and clogged so as to enable a stable casting, by means of forming a glass layer on the surface of the inner bore of the nozzle when the nozzle is used, thereby preventing air from being entrapped through refractory structure thus not to produce alumina, and in addition, smoothing the surface of the inner bore of the nozzle.
The first embodiment of the present invention is a continuous casting nozzle for casting molten steel, wherein at least a surface layer of an inner bore of said continuous casting nozzle contacting with a molten steel is formed of a refractory comprising:
an aggregate consisting essentially of alumina (Al2O3) or an aggregate comprising alumina (Al2O3) as its main ingredient and melting point thereof being at least 1800 degree centigrade (xc2x0C.): from 15 to 60 wt. %; and
roseki as a balance.
The second embodiment of the present invention is a continuous casting nozzle for casting molten steel, wherein at least a surface layer of an inner bore of said continuous casting nozzle contacting with a molten steel is formed by a process in which binder is added to a refractory material comprising 15 to 60 wt. % of an aggregate consisting essentially of alumina (Al2O3), or an aggregate comprising alumina (Al2O3) as its main component and melting point thereof being at least 1800xc2x0 C. and roseki as a balance, and then said refractory material with said binder added is kneaded, formed, and sintered in an anti-oxidizing atmosphere.
The third embodiment of the present invention is a continuous casting nozzle for casting molten steel, wherein a mixing weight ratio of roseki having average grain diameter of up to 250 xcexcm is up to 60 wt. % relative to a total content of said roseki.
The fourth embodiment of the present invention is a continuous casting nozzle for casting molten steel, wherein said roseki comprises pyrophyllite (Al2O3.4SiO2.H2O) as its main component.
The fifth embodiment of the present invention is a continuous casting nozzle for casting molten steel, wherein said roseki comprising roseki which is calcinated at a temperature of at least 800xc2x0 C. so as to remove crystal water therein.
The sixth embodiment of the present invention is a continuous casting nozzle for casting molten steel, wherein said binder comprises a thermosetting resin.